The present invention relates to a variable or adjustable speed pumping-up generator having a frequency converter as an AC exciting device and having a primary circuit synchronously connected to a commercial power supply system in spite of variable speed rotation of a rotating rotor of a the generator, and more particularly to a variable speed pumping-up generator which controls the speed on the pump turbine side in the generation mode. More specifically, the present invention is to permit stable continuous operation without departing from a predetermined variable speed range.
For the generation mode of a variable speed pumping-up generator for the pump turbine, there are two major control systems depending on whether speed control and output control are assigned to the pump turbine or the prime mover side or assigned to the generator motor or the load side.
In the first control system, the generator motor is assigned for the speed control and the pump turbine is assigned for the output control. It is characterized by:
a. Greater importance is put on the speed control and the speed control waveform is complete. PA1 b. Since the electrical power from the generator motor is almost a duplicate of the mechanical power output from the pump turbine, the affect of the water hammer phenomena upstream and downstream the pump turbine is directly appears on the electrical power from the generator motor and even a trend of back swing appears transiently. PA1 a. Rapid and straight-forward power response which is very close to the power command is expected. PA1 b. Since the speed control is conducted on the prime mover side, safety is higher than that in a system in which the speed control is conducted on the generator motor side, or the load side. PA1 first speed control means for outputting a rotating speed command signal, in the generation mode, for controlling said pump turbine in accordance with a power output command and outputting an AC exciting control signal to said AC exciting control unit; and PA1 second speed control means for controlling, in the generation mode, an amount of AC excitation from said AC exciting unit when the rotating speed of said rotary shaft falls below a lower limit setting; PA1 a distance between the lower limit setting and a lower limit of a normal rotating speed adjustable range of said shaft being set to be larger than at least one half of a peak-to-peak amplitude of possible sustained speed oscillation, say, 0.15 percent of the rated speed which corresponds to no slippage or zero excitation frequency in a case where sustained speed oscillation is expected to be. PA1 first speed control means for outputting a rotating speed command signal, in the generation mode, for controlling said pump turbine in accordance with a power output command and outputting an AC exciting control signal to said AC exciting control unit; and PA1 second speed control means for controlling, in the generation mode, an amount of AC excitation from said AC exciting unit when the rotating speed of said rotary shaft rises above an upper limit commencement setting; PA1 the upper limit commencement setting being higher than a maximum value of a normal adjustable speed range in the generation mode and lower than an upper limit of the normal adjustable speed range in the pumping up mode.
As a result, the resultant adverse affect to the connected commercial power supply system is a problem, and this control system is not recommendable.
Thus, the second control system in which the pump turbine is assigned for an optimum speed control in accordance with an external power command signal, and the generator motor is assigned for a power control to cause the power to directly follow an external power command. It is characterized by:
In this system, even if a rapid power change command is given, the adverse affect from the water hammer phenomena does not appear on the commercial power supply system at all.
However, since the speed control depends on the flow rate control of the pump turbine which has a slow response, the speed tends to respond too slowly or to allow overshooting to some extent.
On the other hand, it is inoperative for the adjustable speed machine to control speed strictly within the predetermined speed range because otherwise the frequency converter lose its function.
Further, in the adjustable speed pumping-up generator, an operating speed range in the turbine mode or the generation mode of the adjustable speed pump turbine is, usually, limited to a lower area of the adjustable speed range while an operating speed range in the pumping-up mode is over the substantially entire range. This is due to the consideration of the fact that the turbine efficiency is higher at a lower rotating speed than the synchronous speed and an input adjustable range in the pumping up mode should be as large as possible.
Thus, in the adjustable speed pumping-up generator, when the rotating speed of the pump turbine overshoots beyond a lower limit by a predetermined amount or beyond an upper limit by a predetermined amount in the generation mode, a backup speed control by adjusting electrical power output of the adjustable speed generator motor comes in service to suppress the overshoot as disclosed in Japanese Patent publication (Kokai) No. JP-A-62-071497.
FIG. 6 shows the system of JP-A-62-071497. An induction motor 1 is rotated by a water turbine 2 directly connected to a rotor thereof, and an AC exciting current controlled to a predetermined phase in accordance with a rotating speed of the induction motor 1 by a secondary exciting control unit 3 having a frequency converter is supplied to a secondary winding lb of the induction motor 1, and adjustable speed operation is conducted such that an AC power having the same frequency as that of an AC system 4 is outputted from a primary winding la of the induction motor 1. A power for controlling the secondary exciting control unit is supplied by a receiving transformer 12.
A function generator 5 receives an external generation output command Po and a water level detection signal H to generate an optimum rotating speed command Na and a feed-forward signal Ya for improving response speed of the guide vanes. When a variation of water level is small, the water level signal H may be omitted. A rotating speed control unit 16 compares an actual rotating speed N detected by a rotating speed detector 6 with the optimum rotating speed command Na to output a guide vane opening control signal .DELTA.Y. The feed-forward signal Ya from the function generator 5 and the guide vane opening control signal .DELTA.Y are summed by an adder 21 and a sum is inputted to a guide vane drive unit 10 which controls the guide vanes 11.
A slip phase detector 7 detects a slip phase Sp which is equal to a difference between a potential phase of the AC power supply system 4 and a secondary rotating phase of the induction motor represented by an electrical angle. A configuration of the slip phase detector 7 is explained. A rotor of the slip phase detector has a three-phase winding connected in parallel to the primary winding la of the induction motor 1, and Hall converters are arranged on a stator of the slip phase detector 7 at positions spaced by .pi./2 electrical angle so that signals having matched voltage phase of the AC system 4 as viewed from the secondary of the induction motor 1 are outputted from the Hall converters and converted to the slip phase Sp, which is inputted to the secondary exciting control unit 3. The rotating speed N detected by the rotating speed detector 6 is inputted to a generation output correction command unit 25, and an output .DELTA.P2 therefrom and the external generation output command Po are summed by an adder 26 which produces an induction motor output command P.sub.G which in turn is inputted to the secondary exciting control unit 3.
A configuration of the generation output correction command unit 25 is explained. When the rotating speed is between settings N2 and N3, the generation output correction command signal .DELTA.P2 is kept at zero, and when the rotating speed N is lower than the setting N2, the generation output correction command signal .DELTA.P2 is decreased in proportion to the decrease of the rotating speed N. On the other hand, when the rotating speed N is higher than the setting N3, the generation output correction command signal .DELTA.P2 is increased in proportion to the increase of the rotation speed N. An absolute value of the generation output correction command signal .DELTA.P2 is controlled not to exceed P2. The settings N2 and N3 are set in accordance with a voltage rating and a frequency output range of the frequency converter of the secondary exciting control unit 3 and a rotating speed range determined by performance characteristics of the turbine 2.
The induction motor output command PG and the slip phase Sp of the slip phase detector 7 are inputted to the secondary exciting control unit 3 and an AC exciting current supplied to the secondary winding lb of the induction motor 1 is controlled such that the output detection signal P of the induction motor 1 detected by effective power detector 9 is equal to the induction motor output command PG. More particularly, the control method disclosed in Japanese Patent publication (Kokoku) No. JP-B-57-60645 may be applied. The guide vane drive unit 10 controls the guide vane 11 in accordance with a sum of the feed-forward signal Ya and the guide vane opening control signal .DELTA.Y to control the water turbine output PT.
A response when the generation output Po is increased stepwise as shown in FIG. 7(a) to increase the generation output P stepwise when the rotating speed N is around the setting N2 at a time t0 is explained. When the generation output Po is increased stepwise at the time t0 as shown in FIG. 7(a) to increase the generation output P stepwise, the generation output P of the induction motor 1 follows the change of the generation output command Po and is increased as shown in FIG. 7(g). 0n the other hand, the response of the opening Y of the guide vanes 11 to the sum of Ya and .DELTA.Y which is mechanically conducted is slower than the response of the generation output P to the generation output command Po. As a result, the water turbine output PT is lower than the generation output P and the rotating speed N is temporarily decreased after the abrupt change of the generation output command Po, and thereafter at a time t1, the generation output P and the water turbine output PT become equal and decreasing of the rotating speed N causes to record a transient minimum value. At the time t1, since the speed error .DELTA.N is positive, the guide vane opening control signal .DELTA.Y is positive and the guide vane opening Y continues to increase. Accordingly, the water turbine output PT becomes larger than the generation output P and the rotating speed N starts to rise as shown in FIG. 7(f). As the rotating speed N increases, the error from the optimum rotating speed command Na decreases, and as the guide vane opening control signal .DELTA.Y decreases, the difference between the water turbine output PT and the generation output P decreases and the acceleration of the rotating speed N decreases.
If the rotating speed N falls below the setting N2 during the decrease of the rotating speed N after the abrupt increase of the generation output command Po, the following occurs. When the rotating speed N decreases below the setting N2, the induction motor output command PG inputted to the secondary exciting control unit 3 is smaller as much as the generation output correction command signal .DELTA.P2 than the external generation output command Po. As a result, the time t1 at which the water turbine output PT becomes equal to the generation output P occurs earlier than the time t2 at which the water turbine output PT becomes equal to the generation output command Po, and the time point at which the rotating speed N record a transient minimum shifts from the time t2 to the time t1 as shown by a broken line in FIG. 9(f) so that a transient overshoot of the rotating speed N is materially reduced. Thus, the backup speed control is conducted such that the rotating speed of the adjustable speed water turbine generator does not lower beyond the predetermined setting value. A similar phenomenon occurs when the upper limit setting N3 is exceeded.
However, the prior art does not teach where the backup speed control through the secondary exciting control unit 3 is to be effected, that is, how the lower limit setting N2 and the upper limit setting N3 are to be set.
Further, it has been proved that the above prior art system is not applicable when the operation speed range in the generation mode is limited to the lower area in the adjustable speed range and a difference between the upper limit of the operation range in the generation mode and the upper limit of the operation range in the pumping up mode is large. Namely, when the backup speed control is set slightly below the lower limit of the generation mode adjustable speed range and slightly above the upper limit of the pumping up mode adjustable speed range, the rotating speed N transiently exceeds the generation mode variable speed range upper limit significantly immediately after the abrupt decrease of the power command Po during the operation near the 100% output in the generation mode. Namely, the suppression effect of the overshoot by the backup control is not available until the rotating speed N reaches its upper limit setting and substantial speed excursion occurs in the adjustable speed range. Even if the upward excursion of the rotating speed N is allowable, because it is still within the adjustable speed range for the frequency converter or it can be easily suppressed by the backup speed control at the upper limit of the adjustable speed range, if the power command decreases abruptly subsequent to the upward excursion and accordingly the rotating speed command also decreases abruptly, the rotating speed rapidly decreases by coincidence of a swing-back of the upward excursion. Consequently an overshoot occurs below the lower limit of the adjustable speed range. It may step into the lower backup speed control range until the suppression effect increases to a extent to overcome the downward overshoot energy. The amount of step-in may be so large that the synchronization is lost.